In a hybrid vehicle, an inverter plays the role of converting high voltage DC power into AC power to operate a motor. This inverter includes an IGBT (Insulated Gate Bipolar Transistor) module (or, FET module). The IGBT module, as shown in FIG. 1a, includes a plurality of switching elements SW1˜SW6 and each of the switching elements SW1˜SW6 is operated by a PWM signal outputted from a driving unit 11. The PWM signal output is controlled by a controller 12.
However, an inverter three-phase output can be short circuited as a result of vehicle accident, aging of the cable, and errors in the assembling process, thereby causing a state in which an excess current flows into the IGBT module 10. If such excess state is caused, the controller 12 stops the switching of the switching elements SW1˜SW6 so as to prevent the damage of the IGBT module 10.
However, in case where the switching of the switching elements SW1˜SW6 is suddenly stopped, an overvoltage is instantaneously caused between the both ends (drain and source or collector and emitter) of the switching elements SW1˜SW6 such that the IGBT module 10 is damaged.
When a circuit is instantaneously short circuited or an overvoltage is generated during the operation of the IGBT module 10 as described above, an overshoot (A) that the voltage VDS applied to the drain end and source end of the switching element SW1 of the IGBT module 10 is instantaneously increased to a value higher than the breakdown voltage of the IGBT module 10 is caused, as shown in the timing diagram of FIG. 1b. The overshoot can damage the IGBT module 10.
To prevent such damage, as shown in FIG. 2a, a clamping unit 20 is inserted between the drain end or collector end and the gate end of the switching element SW1, so that the overvoltage can be lowered by the clamping unit 20.
The clamping unit 20 includes a clamping diode D1, a reverse-current prevention diode D2 and a resistance R1. If the breakdown voltage of the clamping diode D1 is set to be lower than the breakdown voltage of the IGBT module 10, and the VDS voltage is increased to a value higher than the breakdown voltage of the clamping diode D1, the clamping diode D1 is turned on, which makes current ID1 flow through the clamping unit 20 so that the switching element SW1 of the IGBT module 10 is turned on. As a result, the VDS voltage is decreased, thereby clamping, as shown in FIG. 2b, the overshoot voltage (B).
Nonetheless, if a voltage greater than the breakdown voltage of clamping unit 20 is continuously applied as the system voltage (DC input voltage) to the IGBT module 10, the switching element SW1 is continuously turned on by the clamping unit 20 and may be overheated.
That is, if it fails to control the inverter while a motor rotates at a high speed, counter electro-motive force of the motor is applied to the IGBT module 10, which then causes the system voltage VDC applied to the IGBT module 10 to be higher than the breakdown voltage Vclamp of the clamping unit 20. Accordingly, the clamping diode D1 is turned on and the switching element SW1 of the IGBT module 10 is continuously turned on. As a result, excess current flows through the IGBT module 10 and the IGBT module 10 may be damaged by overheating.
As described above, if the breakdown voltage of the clamping diode is set to be lower than the system voltage such that the system voltage is increased to a value higher than the breakdown voltage of the clamping diode D1, the IGBT module may be damaged.
One approach to solve this problem was to increase the breakdown voltage of the IGBT module and the breakdown voltage of the clamping diode so as to increase the system voltage. However, increasing both the breakdown voltage of the IGBT module and the breakdown voltage of the clamping diode caused the price of the IGBT module to be increased.
For these reasons, it is difficult to achieve a maximum performance of a vehicle motor. Moreover, it is difficult to use an IGBT module having a certain breakdown voltage commonly for various vehicles since different vehicles require different the system voltages (DC input voltages).
The above information disclosed in this the Background Art section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.